Principles of Mounting Quartz Plates

I T IS the object of this chapter to show some of the fundamental considerations involved that govern the design of mountings or holders of quartz crystals. This discussion is restricted to the three common types, namely, rod or clamp type, wire type and airgap type. The development of these three types of mountings for applications in telephone transmission and radio systems has led to many and varied forms. Commercial designs of units for telephone uses employing these principles are described in detail in a later chapter. In chapter VI regarding the vibrations of crystals we have assumed in all cases that the crystal is free to vibrate. In order that this condition shall be fulfilled it is necessary that any mounting which supports the crystal shall not restrict its vibration or at most the effect shall be made as negligible as possible. 8.1 CLAMP T Y P E SUPPORTS Of the known types of vibration it is noticed in all cases that there have been nodal points. These points by definition are points of zero motion and in all cases that we have studied appear to be single isolated points or lines of very small size in comparison with the total crystal area. The obvious type of mounting is then one which simply clamps the crystal with a very small area at these points or nodes. The early type of mountings for low-frequency crystals were all based on this principle and the area of the clamp was determined experimentally by reducing it until, with sufficient pressure to hold the crystal, a good Q was obtained.